Tooling Up for Fluid-End Machining

Demand for fluid ends is rising because of increased drilling and the component’s short lifespan.

Directional drilling, or fracking, for oil and natural gas layered within shale deposits is possibly the hottest trend in today’s energy industry. Critical to that drilling process, especially for natural gas, are fluid ends that regulate liquid solutions as they are pumped into a well. So as more wells are drilled, the demand for fluid ends continues to surge. And magnifying that demand is the fact that the lifespan of a fluid end, at most, is a couple of weeks, or in some instances only a few days.

Fluid ends are big vessels made completely from solid blocks of steel, and they require huge amounts of machining. Milling, drilling and threading are the major machining operations within the three main phases of processing a fluid end—cutting and cubing, rough machining and finish machining.

Fluid ends pose several machining challenges, including raw material scale, varying depths-of-cut and long tool overhangs during milling operations. While in drilling and threading operations, holes can measure up to 8" (203 mm) in diameter and are extremely long—some taking 40 minutes in the cut to complete. Plus, they typically intersect with three to five crossholes, much like on automotive engine blocks. It is for these machining challenges that tooling companies, such as Seco, have turned their focus to fluid ends and have developed tooling systems specifically for optimizing each of the three phases of processing those components.

Phase 1: Cutting and Cubing

When rough billets for fluid ends arrive from foundries, they must first be squared up or cubed. This operation prepares the block for rough machining. It is during the cubing process when scale on billet surfaces and inconsistent depths-of-cut come into play.

Cutters must be tough for this processing phase. And to cost-effectively increase productivity it is recommended to use free-cutting face mills that allow for high metal removal, yet long tool life. Cutters should also incorporate multiple inserts with multiple available cutting edges and allow for using the same tool for both roughing and finishing.

In Seco’s case, its Double Octomill provides 16 cutting edges and positive insert geometries. But the main advancement of the mill is its precision insert seating technology. A set of hardened high-speed-steel pins in each insert pocket locate the insert accurately and securely. This makes them easy to index, and helps to reduce fluid machining time, as it also eliminates having to make axial adjustments.

Phase 2: Rough Machining

About 40% of material is machined from fluid ends during rough machining, which entails milling, drilling and thread milling. To optimize these operations and reduce cycle times, tooling must provide the highest metal removal rates possible, as well as have the strength and stability for long-reach situations.

For rough milling, high-feed-type milling cutters are recommended. They must be stable enough for extremely aggressive face milling or helical interpolation to rough bore holes. Also, toolholders that drastically improve the dynamic rigidity of milling assemblies are required. They will allow for increased cutting speeds and feeds while optimizing stability to make for quieter and vibration-free cutting, and thus improved surface finishes.

When working with long tool overhangs in milling, as is often the case with fluid ends, a major problem is vibration, which is usually dealt with by slowing down machining speeds and feeds. But that leads to loss of productivity and reductions in tool life.

Passive dynamic-type vibration-dampening toolholders, such as those developed by Seco, will eliminate vibration and the instability it causes. The holders let shops run at higher cutting conditions to increase productivity while improving surface finishes, extending cutting tool life and preventing damage to machine tool spindles as a result of excess vibration.

At the base of fluid ends are a series of mounting holes. And for these holes, scalloped clearance features must be machined using special disk milling cutters. For its customers, Seco will custom engineer its disk mills not only to meet specific fluid end requirements, but also to allow for multitasking operations to, again, reduce cycle times.

Modular drilling systems offer the most advantages in fluid-end machining. In addition to versatility and wide ranges of adjustment capabilities, modular systems make it possible to perform multiple operations with one tooling system—for instance, pilot drill, drill and countersink all at once and with the same tool. With the operational flexibility of one modular drilling system, shops machining fluid ends are able to drill, mill countersinks using circular interpolation, plunge, bore and accurately drill holes in angled surfaces in addition to easily drilling across existing holes. Modular systems reduce the amount of required tooling needed for holemaking operations in fluid ends. Less separate tooling means fewer tool changes, and fewer tool changes help shorten part cycle times.

Specifically for efficiently producing large, deep holes, such as those found in fluid ends, Seco is currently developing an innovative holder for its modular drilling system. This long-reach holder will resemble and work much the same as an auger. It will be very rigid, quickly channel away chips and allow shops to drill holes up to 30" (762-mm) deep. With such a system, the often 57" (1448-mm) deep through holes in fluid ends could be accurately generated by drilling from opposite part ends.

As opposed to tapping, Seco advises that shops use thread milling for generating ID, as well as OD, thread patterns. Thread forming toolholders designed with multitooth cutter bodies, unlike taps, can machine threads to a hole’s full depth, even in materials that have already been hardened. And they do so without sacrificing thread form, surface finish or accuracy.

Phase 3: Finish Machining

The finishing phase is the most critical part of fluid-end machining. Finish machining, along with rough and semifinish machining, of holes is especially important and can be a costly process. Highly productive boring tools, for instance, that can perform each stage are imperative for not only reducing costs but also for ensuring process reliability.

Phase 3 also encompasses threading operations that are the final steps of component processing. Any errors in generating thread profiles at this stage means the fluid end is completely scrapped, at huge losses in time and money. In the finishing stages of fluid ends, process stability and accuracy are mandatory. However, precise threading and boring applications can be time consuming, so only the most accurate and productive tooling should be used.While many shops attempt to produce accurate threads using general-purpose thread mills, Seco has recently developed a new spiral indexable thread mill specifically for generating American Buttress thread profiles for fluid ends. This high-performance mill makes for smoother threading operations while allowing for higher feed rates to shorten cycle times.

The new system uses eight high-accuracy form-ground inserts in a cutter that holds them securely for better gaging of threads. A helical system design reduces cutting tool pressure to eliminate chatter and boost thread form precision. Additionally, the thread mill provides efficient chip evacuation and requires less machine tool horsepower.

For semifinish and finish boring fluid-end holes, boring heads must incorporate rigid and accurate connections that allow shops to add extensions for long reach. And even more advantageous are boring systems that provide Capto-style connections so the tools can be used on multitasking machines.

To increase metal-removal rates in semifinishing, twin bore heads are recommended. These types of boring heads provide both symmetrical and staggered operations and settings. With Seco’s system, for instance, shops have the ability to use both 80° and 90° lead-angle inserts or run double-sided negative inserts.

In hole finishing, single-tooth/insert-finish boring heads should be used for holding tight tolerances. The heads must be able to consistently generate the IT5 hole tolerance required on most fluid ends by providing micrometer adjustments down to 0.0001" (0.0025 mm), as do Seco’s finish boring heads.

The high boring speeds and long tool overhang conditions in fluid-end machining require that finish boring heads are capable of being balanced. Plus, when using larger finishing heads with extensions, as is also the case in fluid-end machining, weight can be an issue for both operators and machine tools. In these instances, lightweight heads, such as those made from aluminum, can reduce weight by as much as 60% over comparable steel setups.

The majority of shale deposits are in regions other than those within typical oil states, and as the need for repair shops situated in close proximity to fracking rigs continues to grow, so too will the demand for fluid ends. To meet these production demands and increase competitiveness, shops must work with tooling providers in an effort to continuously incorporate the latest most advanced tooling developed specifically for their applications, such as the processing of fluid ends.

This article was first published in the 2013 issue of the Energy Manufacturing Yearbook.